1. Field of the Invention:
The present invention relates to a source for producing broadband radiation, and, more specifically, to a tunable source for producing broadband radiation in the millimeter wavelength range of the spectrum.
2. Description of the Prior Art:
Certain analysis or detection schemes operate on the principle that particular objects or substances absorb radiation at particular frequencies or groups of frequencies. For example, a particular type of contraband substance might absorb radiation at discrete frequencies, but not at others. This frequency absorption pattern can be used as a fingerprint to identify the particular substance. Analysis or detection schemes based on this principle must necessarily be able to produce a wide range of selectable frequencies to be applied to the object from a broadband source of radiation. The present invention is directed towards a tunable broadband source having this capability.
In the prior art, gyrotrons have been used to produce broadband radiation in the millimeter wavelength portion of the spectrum. However, gyrotrons are extremely large and bulky, and produce megawatts of power. Such units are entirely impractical for use outside the laboratory, in applications such as airport detection equipment, or even for most medical diagnostic applications.
A backward wave oscillator may also be used to obtain broadband radiation in the desired millimeter range by coupling such a device with frequency multipliers, such as doublers or triplers. The problem with such an arrangement is that the frequency multiplication is very inefficient, and the total output power is less than 1 milliwatt. Backward wave generators are also quite expensive.
A klystron can also be used, in combination with a series of multipliers, to generate a plurality of selected frequency bands in the desired range. However, klystrons, like gyrotrons, are not readily tunable, and thus the number of frequencies which can be selected in the millimeter range is quite limited.
Similarly, certain lasers output a large number of lines in the desired millimeter wavelength range, but these lines, being discrete, do not necessarily correspond to the desired frequencies of interest.
Various solid state emitters such as Gunn diodes and IMPATT emitting diodes also emit radiation in discrete bands, but only at the lower end of the millimeter range. The desired radiation for analysis and detection is generally in the higher millimeter wavelength bands. Again, such devices have limited use in analysis and detection applications because they do not emit broadband radiation.
Simple spark gap devices generate electrical sparks that emit radiation over the entire spectral range. However, the radiation emitted from such devices is very low power and contains excessive interference, thus making the use of spark gaps unmanageable as radiation sources.